This invention relates broadly to Sheet Feeders, and more specifically to Sheet Feeders of a type for feeding individually, forward-most sheets, from a pile, usually to clamps of endless-chain conveyors.
It should be understood that "sheets" as used herein refer to envelopes as well as to individual sheets and other thin elements.
Reciprocating vacuum shuttle-plate sheet feeders are well known, with several being shown in U.S. Pat. Nos. 3,844,551 to Morrison and 4,657,236 to Hirakawa et al. A cycle of operation for these vacuum-type sheet feeding devices is normally approximately as follows: A suction is applied through a shuttle plate to a forward-most sheet in a sheet stack, thereby adhering the forward-most sheet to a sheet-engaging surface of the shuttle plate. The shuttle plate then moves in a feed direction carrying the forward-most sheet with it below a rigid blocking gate and delivers this to rollers, or additional conveyors, which then pull the sheet the rest of the way from the stack. At this point, the suction is turned off and the shuttle plate returns to its normal position at the sheet stack.
In some such systems, elongated knife gates, or other blocking structures, are used to restrain other sheets in the sheet stack from moving with the shuttle plate, while in some such systems the sheet stack rests on one or more ledges from which the bottom sheet is pulled prior to being fed forwardly by the shuttle plate. The sheet shuttle feed described in U.S. Pat. No. 3,844,551 to Morrison, combines both of these features. In this regard, one difficulty with some prior-art vacuum reciprocating-shuttle-plate sheet feeding devices is that suctions, or partial vacuums, applied by shuttle plates thereof, bleed through forward-most sheets and cause second-from-forward sheets to adhere to the forward-most sheets. When this happens, two sheets are sometimes fed forwardly by the shuttle plates. It is possible to reduce such "double feeds" by reducing the amount of suction applied to the forward-most sheets; however, such a method also reduces the strength with which the shuttle plate holds the forward-most sheet. This sometimes produces "miss feeds," that is, a shuttle stroke that feeds no sheet. By having redundant separators, U.S. Pat. No. 3,844,551 to Morrison allows a sufficiently high vacuum for substantially reducing "miss feeds" while preventing "double feeds" by pulling corners of forward-most sheets from a ledge with a separate suction cup just prior to their being fed. However, this redundancy has a price inasmuch as the structure required to move the separate suction cup is an added expense and its operation causes additional vibrations during overall operation of the sheet feeder. Thus, it is an object of this invention to provide a sheet feeder which reduces the number of "double feeds" and "miss feeds" but yet which does not require the use of a separate reciprocating sheet separator prior to or during movement of a vacuum shuttle plate.
There is a difficulty in positioning blocking structures in vacuum shuttle-plate sheet feeders employing such blocking structures, or gates, to prevent other sheets from following forward-most sheets. That is, if a blocking structure is positioned too high relative to its shuttle plate, it may allow a second-from-forward sheet to follow the forward-most sheet and if it is too low, it may improperly prevent a thick forward-most sheet from being fed. This problem is magnified when the sheet feeder is used for feeding envelopes. In this regard, it is difficult to separate a forward-most envelope with a throat knife, or other blocking structure, because loose envelope edges and windows tend to catch on the knife. For this reason, when feeding envelopes, it is desirable to have such a blocking structure, or throat knife, in a relatively open position. On the other hand, when such a throat knife is too "open" a double will occur. It is an object of this invention to provide a vacuum shuttle-plate sheet feeder in which a throat knife can be placed in a relatively "open" position so that it can be easily used with envelopes but yet which does not produce an undue number of "double feeds".
Yet another difficulty with reciprocating vacuum shuttle plate sheet feeders has been that shuttle plates thereof applied suction at fixed locations on forward-most sheets. For example, in a device of U.S. Pat. No. 3,844,551, a suction groove of a shuttle plate thereof is in one position relative to the shuttle plate and a hopper and cannot be moved. A difficulty with such a structure is that a position of its suction groove cannot be tailored to fit different size and shaped envelopes. Thus, the vacuum groove may damage envelope windows if its necessary position happens to coincide with envelope windows, for example. Also, suction-groove positioning may detrimentally affect the accuracy of sheet feeds because when a sheet is pulled too close to an edge thereof it often skews and jams, especially when there is a heavy stack of paper thereon. Ideally, a suction groove should be arranged to pull a sheet at a position as close to the center thereof as possible. It is therefore, an object of this invention to provide a vacuum shuttle plate sheet feeder in which the position at which vacuum is applied relative to a hopper and a shuttle plate can be varied so that the shuttle plate can be tailored to fit various size and shaped envelopes.
Many previous vacuum shuttle sheet feeders feed envelopes directly to indexed, or momentarily stationary, clamps mounted on endless chain conveyors. Any slippage in conveying such envelopes from bottoms, or forward-most positions, of their stacks to the gripper jaws of such clamps causes an imprecise placement of the envelopes in the jaws which often causes jams or improper feeds downstream thereof. For example, if an envelope is crammed too firmly into a gripper jaw of a clamp, a leading edge of the envelope will be bent, thereby causing problems for later handling of the envelope. On the other hand, if the envelope is not fed far enough into a clamp's gripper jaw, the envelope might be inadequately held when the gripper jaw closes, again causing problems downstream. Thus, it is an object of this invention, to provide a transition structure between a sheet feeder and a gripper jaw of an endless conveyor mounted clamp such that a sheet is fed precisely into the gripper jaw thereof.
Yet another difficulty with many sheet feeders is that stacks placed in hoppers thereof cause great weight forces pressing downwardly on bottom, or forward-most, sheets therein, making it difficult for shuttle plates and the like to pull these forward-most sheets from the stacks. It is an object of this invention to counteract this downward gravity force acting on forward-most sheets to thereby make it easier for shuttle plates, and similar separating elements, to pull forward-most sheets from stacks.